Gold ARVs – and other medicinal metals – lead to research award

OCTOBER 1, 2015

Anti-retroviral drugs made from gold? It’s a possibility, says Nelson Mandela Metropolitan University’s Prof Zenixole Tshentu, who has been using South Africa’s abundant metals to formulate new drug compounds.

His pioneering research into the separation of metal ions is extending boundaries in terms of how these important elements can be used, including in the realm of medicine. Over the past few years, Tshentu and his post-graduate students have been focusing on gold anti-retrovirals and the use of vanadium to treat diabetes.

The inorganic/analytical chemistry associate professor – who is the recipient of one of two NMMU Research Excellence Awards – is essentially “upgrading” the country’s raw mineral resources by improving the methods used to purify them. In so doing, he is increasing their value.

Tshentu says the purification techniques used to upgrade these metals for new uses can be used just as effectively to recycle the precious metals found in discarded goods, such as those contained in the catalytic converters of cars (in exhaust systems), or inside discarded computer chips, referred to as e-waste.

“I call it urban mining. Our minerals underground are drying out – the platinum-group metals – yet the world is only recycling 17% of its precious mineral resources. These resources are not going to be around forever.”

This innovative researcher chooses the “hydrometallurgical route” when it comes to purifying precious metals. This involves creating water-based chemical solutions, in which reagents (substances used in chemical reactions to selectively extract metal ions) are developed to target specific ions.

The chemical system is first modelled computationally. “This allows us to pre-test the interactions that we think are critical to achieve selective separation.”

Once this is achieved, the functional chemistry is transferred onto a solid support (Tshentu and his students typically use polymer nanofibre materials as well as microspheres and silica particles) for ease of separation.

These solid phase-based separation systems are also suitable for processing low concentrations of precious metals and still achieving the full recovery of the metals. Hence, they are suitable for the urban mining of precious metals or for processing low grade ores.

Tshentu said Lonmin, the world’s third-largest platinum producer, had shown interest in NMMU’s purification/separation technology, and was testing one resin that was developed by a PhD student.

In driving towards applications, Tshentu and his students have developed vanadium complexes that could be used as an alternative to chromium supplements (which are already on the market) to treat type 2 diabetes. “Vanadium inhibits one of the enzymes that hampers the internalisation of glucose within cells.” Tshentu said though vanadium and chromium could both only be taken in very small quantities due to their toxicity levels, vanadium appeared to be more effective than chromium in the treatment of diabetes. He said only two vanadium compounds were currently being tested in clinical trials and there was scope for further development.

Tshentu has also contributed to the use of vanadium in the “oxidative desulfurisation of fuels”, which he explained as the conversion of sulfur compounds in fuel so that they can be easily removed (as polar sulfones).

In explaining his research on gold, Tshentu said platinum compounds were already on the market to treat cancer, through chemotherapy. “Scientists have also designed gold compounds that work in a similar manner to platinum to target DNA and some that work on other targets other than DNA.” Now, they’re looking at using gold to treat HIV/Aids.

“What is being looked at is the effect of gold on HIV1 protease [one of the enzymes that are essential for the lifecycle of HIV]. Current drugs work on inhibiting the active site of protease thereby inhibiting the growth of the virus, whereas gold deforms the site by binding to selenium residues on the HIV protease.”

Tshentu and one of his PhD students have constructed molecules with a dual effect, where both gold and its carrier ligand (a molecule attached to the gold atom) act separately but achieve a synergistic effect. “We’ve designed ligands that fit the active protease site, but we have bound them to gold with a low level of affinity so that the gold can break off and bind to the protein. The ligand would then go into the active site and inhibit the interaction of the protease with the natural substrate. We are still trying to prove this.”

“It’s not easy to make gold compounds because of the reactivity of gold ions ... The reactions are not predictable.”

When it comes to building inorganic compounds, Tshentu is passionate about “getting the design right”, especially when it comes to bio-inorganic compounds that could potentially be travelling through the body, and surviving different environments, before reaching their target.

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